Organize D&D extensions by domain

docs/internal/llm.md

@@ -80,7 +80,9 @@ configuration facts in [Configuration](../config.md#defaults).
 `AssetRegistry` combines caller-owned prompt filesystems under stable prefixes
 and rejects invalid or conflicting registrations. Production module packages
 register their own prompt and schema assets; generic framework code contains no
-D&D prompt content.
+D&D prompt content. `internal/framework/promptfs` provides the domain-neutral
+filesystem composition helper used to combine module-owned files with shared
+domain prompt fragments.
 
 Schema helpers load embedded JSON Schema with identity and digest metadata,
 return defensive copies, and expose a diagnostics map that omits schema bytes.